arXiv subjects

Sungjin Park

Publications and source records attributed to Sungjin Park.

22 records · Page 2Linked to original sources

arXiv2020

Semi-supervised Disentanglement with Independent Vector Variational Autoencoders

We aim to separate the generative factors of data into two latent vectors in a variational autoencoder. One vector captures class factors relevant to target classification tasks, while the other vector captures style factors relevant to the remaining information. To learn the discrete class features, we introduce supervision using a small amount of labeled data, which can simply yet effectively reduce the effort required for hyperparameter tuning performed in existing unsupervised methods. Furthermore, we introduce a learning objective to encourage statistical independence between the vectors. We show that (i) this vector independence term exists within the result obtained on decomposing the evidence lower bound with multiple latent vectors, and (ii) encouraging such independence along with reducing the total correlation within the vectors enhances disentanglement performance. Experiments conducted on several image datasets demonstrate that the disentanglement achieved via our method can improve classification performance and generation controllability.

cs.LG↗

arXiv2019

Hybrid-functional and quasi-particle calculations of band structures of Mg2Si, Mg2Ge, and Mg2Sn

We perform hybrid functional and quasi-particle band structure calculations with spin-orbit interaction to investigate the band structures of Mg2Si, Mg2Ge, and Mg2Sn. For all Mg2X materials, where X = Si, Ge, and Sn, the characteristics of band edge states, i.e., band and valley degeneracies, and orbital characters, are found to be conserved, independent of the computational schemes such as density functional generalized gradient approximation, hybrid functionals, or quasi-particle calculations. However, the magnitude of the calculated band gap varies significantly with the computational schemes. Within density-functional calculations, the one-particle band gaps of Mg2Si, Mg2Ge, and Mg2Sn are 0.191, 0.090, and -0.346 eV, respectively, and thus severely underestimated compared to the experimental gaps, due to the band gap error in the density functional theory and the significant relativistic effect on the low-energy band structures. By employing hybrid-functional calculations with a 35% fraction of the exact Hartree-Fock exchange energy (HSE-35%), we overcame the negative band gap issue in Mg2Sn. Finally, in quasi-particle calculations on top of the HSE-35% Hamiltonians, we obtained band gaps of 0.835, 0.759, and 0.244 eV for Mg2Si, Mg2Ge, and Mg2Sn, respectively, consistent with the experimental band gaps of 0.77, 0.74, and 0.36 eV, respectively.

cond-mat.mtrl-sci↗

arXiv2015

Quantitative Morphological and Biochemical Studies on Human Downy Hairs using 3-D Quantitative Phase Imaging

This study presents the morphological and biochemical findings on human downy arm hairs using 3-D quantitative phase imaging techniques. 3-D refractive index tomograms and high-resolution 2-D synthetic aperture images of individual downy arm hairs were measured using a Mach-Zehnder laser interferometric microscopy equipped with a two-axis galvanometer mirror. From the measured quantitative images, the biochemical and morphological parameters of downy hairs were non-invasively quantified including the mean refractive index, volume, cylinder, and effective radius of individual hairs. In addition, the effects of hydrogen peroxide on individual downy hairs were investigated.

physics.bio-ph↗

arXiv2015

Interlayer correlation between two $^4$He monolayers adsorbed on both sides of $α$-graphyne

Path-integral Monte Carlo calculations have been performed to study the $^4$He adsorption on both sides of a single $α$-graphyne sheet. For investigation of the interlayer correlation between the upper and the lower monolayer of $^4$He adatoms, the $^4$He-substrate interaction is described by the sum of the $^4$He-C interatomic pair potentials, for which we use both Lennard-Jones and Yukawa-6 anisotropic potentials. When the lower $^4$He layer is a C$_{4/3}$ commensurate solid, the upper-layer $^4$He atoms are found to form a Kagomé lattice structure at a Mott insulating density of 0.0706 Å$^{-2}$, and a commensurate solid at an areal density of 0.0941 Å$^{-2}$ for both substrate potentials. The correlation between upper- and lower-layer pseudospins, which were introduced in Ref. [1] for two degenerate configurations of three $^4$He atoms in a hexagonal cell, depends on the substrate potential used; With the substrate potential based on the anisotropic Yukawa-6 pair potentials, the Ising pseudo-spins of both $^4$He layers are found to be anti-parallel to each other while the parallel and anti-parallel pseudo-spin alignments between the two $^4$He layers are nearly degenerate with the Lennard-Jones potentials. This is attributed to the difference in the interlayer distance, which is $\sim 4$ Å~ with the Yukawa-6 substrate potential but as large as $\sim 4.8$ Å~with the Lennard-Jones potential. [1] Y. Kwon, H. Shin, and H. Lee, Phys. Rev. B 88, 201403(R) (2013)

cond-mat.mtrl-sci↗